Some temporary foreign workers, mostly from China and Korea as well as British backpackers, get away with using each others' safety induction cards because subcontractors fail to check their credentials or turn a blind eye to keep costs down, the Construction, Forestry, Mining and Energy Union says.

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''The Immigration Department is absolutely overwhelmed with this,'' Mr Tulloch said. ''Forget about boat people - there are more people coming through the turnstiles at Kingsford-Smith Airport who are then working illegally.''

A schedule seen by the Herald shows the union has recently recovered hundreds of thousands of dollars from construction companies on Sydney sites. It recovered $129,600 for 95 Chinese workers not paid for six weeks at a Lane Cove site. It recouped a further $100,000 from a Rhodes job under the same subcontractor.

A CFMEU state organiser, Mark Cunningham, said he knew of several cases of Sudanese being paid as low as $10 an hour. ''There seems to be large increases of Chinese, Korean and Sudanese being abused.''

In January the union reclaimed $33,000 for seven Chinese workers from Jin Cheng Pty Ltd. The company said most of its workers stayed for only ''two or three weeks'' and their visas were not checked.

Last year six illegal Malaysian tilers were arrested at a Bovis Lend Lease apartment site in Pyrmont. Three were using other people's induction cards and were on tourist visas.

The subcontractor, tiler Saba Bros, told the Herald it had been unaware they were illegal. ''Because of the size of the projects we actually engage sub-sub contractors and sometimes it is beyond our control,'' Saba's general manager, Felix Sudarto, said.

Another raid last year at a Hutchinson Builders apartment site in Dee Why caught up to nine illegal Chinese gyprockers and tilers, the union says.

The regional manager of Hutchinson NSW, Brian Hood, confirmed ''several'' foreign workers were detained but some were released because they were found to be legal.

Construction is the second-highest employer of illegal workers. The Department of Immigration said in 2009-10 it conducted 3752 ''compliance field activities'' relating to illegal workers.

''Compliance officers continue to take strong action against foreign nationals working illegally,'' a spokeswoman said.

A report commissioned by the government said there were about 100,000 illegal workers in Australia.

Jim Barrett, the executive director of the Australian Constructors Association, said illegal workers was not a major issue. ''If those instances are occurring they are less likely to be occurring on the sites of major contractors.''

Ken Phillips, the executive director of Independent Contractors Australia, said: ''I talk to officers [doing raids] and they tell me that it's like squeezing a balloon. You catch one and you know there are 100 out there you haven't caught.''